All of these are real possibilities. There's also one other possibility that was alluded to by several...when one is panicky (or anxious, as when watching your buddy bolt for the surface from 130 ft), the tendency is to take very shallow breaths (both inhaling and exhaling). The lungs never completely evacuate the air from the previous breaths, and as a result the diver is attempting to breathe in more air, when, physically, no additional space to allow such a breath exists. The diver feels starved for air because he or she cannot get a full breath, but the lungs are actually full. It happens a lot in training situations, especially on that pesky mask clearing drill. Instructors see it all the time. Any of these things could be the cause, and I agree that you will probably never know the actual reason. But if the regulator was reasonably tuned, there is no reason to believe that it was a reg malfunction. My advice? Buy your own regulator, pay the luggage fees on the plane, and breath easier!